The past few years has witnessed an almost exponential growth in the extent of high-speed data networks, and the data transmission speeds contemplated over such networks. In particular, bidirectional data transmission in accordance with the various Ethernet network protocols, over unshielded twisted pair (UTP) wiring, has emerged as a network implementation of choice for general commercial LAN installations as well as for some of the more prosaic residential and academic applications.
Local Area Networks (LAN) provide network connectivity for personal computers, work stations and servers. Ethernet in its original 10BASE-T form, remains the dominant network technology for LANs. However, among the high-speed LAN technologies available today, fast Ethernet, or 100BASE-T, has become the leading choice. Fast Ethernet technology provides a smooth, non-disruptive evolution from the 10 megabits per second (Mbps) performance of 10BASE-T to the 100 Mbps performance of 100BASE-T. The growing use of 100BASE-T connections to server and desktops is creating a definite need for even higher speed network technology at the backbone and server level.
The most appropriate solution to this need, now in development, is Gigabit Ethernet. Gigabit Ethernet will provide 1 gigabit per second (Gbps) bandwidth with the simplicity of Ethernet at lower cost than other technologies of comparable speed, and will offer a smooth upgrade path for current Ethernet installations.
No matter how implemented, Ethernet transceivers are found in a very wide range of application installations, from individual notebook or laptop computers to enterprise-wide wiring closets. Because of the range of installation types, Ethernet circuits often face certain conflicting constraints on their I/O modalities. For example, in enterprise-wide wiring closets, where transceiver density is high, radiative emissions (EMI) from constituent transceivers is a major source of concern in the struggle to maintain bandwidth and signal quality by minimizing cross-talk and other extraneous noise sources.
Conversely, in low density installations such as laptop or notebook computers, radiative emissions are of a substantially lower concern while the power consumption of an Ethernet transceiver becomes paramount. Given the relatively limited battery life of a laptop or notebook computer, it will be evident that the constituent electronic components of such a computer must operate at the lowest possible power consumption consistent with proper performance.
A conflict arises when it is recognized that radiative emissions are reduced when a differential signal transmitter, such as an Ethernet transmitter, is transmitting a differential signal in what is termed Class-A mode, i.e., the differential mode current varies in order to define the signal, while the common-mode current component is kept constant. However, constant common-mode current compels such circuitry to conduct a constant quanta of current at all times, even when the differential mode signal defines a zero value. It is well understood that current mode transmitters, outputting a constant common-mode current, necessarily consume relatively large amounts of power, caused by constant conduction of the output section. It is further understood that in order to minimize constant current conduction and thus power consumption, a differential signal system could be operated in what is termed a Class-B mode, i.e., one in which the common-mode current is allowed to vary between some maximum value and zero. However, when operating in Class-B mode, the variable common-mode current causes the very radiative emissions that one would seek to avoid in a high density installation.
Notwithstanding the foregoing, any modern nexus Ethernet communication system should also be able to communicate with other systems across all of the Ethernet communication standards prevalent in today's network environments. The difficulty here is that the different Ethernet standards often have differing differential voltage swing requirements, making the implementation of a cross-standard transmitter platform very difficult. The most common prior art solution to this difficulty is to implement an Ethernet transceiver with multiple transmitter sections, with each transmitter section optimized for performance under a particular standard, i.e., 10BASE-T, 100BASE-T, and the like. While effective in some degree, this particular approach compels a high degree of complexity and duplication in a given transceiver system, as well as requiring a great deal of silicon chip real estate when a cross-standard transceiver is implemented as an integrated circuit. Such solutions are expensive, unstable and, because the large amounts of duplicative circuitry must remain powered-up in order to be available at need, they consume prohibitive amounts of power even when major portions of the circuitry are quiescent.
It would be beneficial, therefore, both to circuit performance and to manufacturing economies, if an Ethernet-capable transceiver were to include a transmitter or transmit DAC that was adaptively configurable to operate as a cross-standard transmitter platform, as well as being adaptively configurable between Class-A and Class-B operational modes, depending on the intended installation. Such a circuit would provide the industry with a single-chip solution having such flexibility that it is able to be incorporated into high density systems where emissions are a problem, as well as low density systems where power consumption is the greatest concern. Such a single-chip solution would be able to communicate with other Ethernet installations regardless of the communication standard chosen.